Numerous bone sites are used as acceptor sites for implant grafting and loading. In dentistry, these sites consist mainly in the mandible and upper or lower maxilla. In orthopaedics, mainly extremities of the femur (thigh bone), humerus or tibia (shinbone) are considered. These bones are composed of two osseous tissues: The dense cortical bone forms the hard outer layer of bone organs. The cancellous bone, or trabecular bone or spongy bone, has a higher surface area but is less dense, softer, weaker, and less stiff. It typically occurs at the ends of long bones, proximal to joints. Its primary anatomical and functional unit is the trabecula. The capacity of these bones to successfully accept an implant depends not only on patient characteristics, surgical technique and implant design but also on bone quality and density and on the structural organization and microstructures of the spongy bone moiety. Chances of satisfactory rehabilitation are based on initial stability of the implant loading as well as on the good biological and biomechanical osseointegration capacity of the implant.
Implant stability is achieved at two levels: the primary stability, which is the mechanical stability obtained immediately after implantation and the secondary stability which is obtained along the osseointegration process. Secure primary stability is both a good indicator and a prerequisite of secondary stability. Being able to assess with accuracy this primary stability as well as the secondary stability enables to design an appropriate surgery protocol and its follow-up.
A major challenge is to develop methodological tools that enable to understand the key elements which contribute to implant performance, in particular regarding implant primary stability.
Primary implant stability refers to the stability of an implant e.g. a dental implant immediately after implantation. Its value is derived from a mechanical engraving typically of a titanium screw implant in the patient's bone tissue. High initial stabilization may be an indication for immediate loading with prosthetic reconstruction.
The value of primary implant stabilization decreases gradually with reconstruction of bone tissue around the implant in the first weeks after surgery, ceding to secondary stability. Its character is quite different from the initial stabilization, because it results from the ongoing process of osseointegration. When the healing process is complete, the initial mechanical stability is fully replaced by biological stability. The most dangerous moment for implantation success is the moment of the lowest initial stabilization, pending sufficient bone reconstruction supporting long-term maintenance of the implant. Usually this occurs during the 3-4 weeks after implantation. If primary stability was not high enough following implantation, the implant's mobility is high and can cause failure.
Resonance frequency analysis (RFA—using the Osstell™ device) and the damping capacity assessment (Periotest™ technique) are the nondestructive intraoral testing methods for assessing implant stability after implantation. In the initial Periotest technology, an electronically controlled rod typically taps the implant a few times per second at a constant speed. The rod is decelerated when it enters in contact with the implant and its frequency is modified. When implants are stable, the deceleration is higher, and so is the damping effect of the tissues surrounding the implant. After hitting the implant, the rod recoils. A faster recoil indicates higher damping. The Periotest™ technology is intended to provide objective implant stability values used for evaluating implant-bone interface stability. Resonance frequency analysis (RFA) is a noninvasive and non-destructive quantitative measurement of implant integration by assessing changes in implant stability over time. This technology consists in the use of an adapter placed on a screw which is attached to the implant. Then a probe emits magnetic pulses at different frequencies that trigger the screw to vibrate. The adapter starts to vibrate, the probe listens to the tone and translates it into the resonance frequency (RF) to which corresponds an ISQ (Implant Stability Quotient) value. The higher the frequency, the more stable the implant is. ISQ is used as a scale that indicates the level of stability and osseointegration in dental implants. The ISQ scale typically ranges from 1 to 100, with the acceptable stability between 55-85 ISQ. In its most recent wireless version, RFA makes use of a magnetic peg—the so-called Smartpeg—attached to an implant or abutment. The peg is excited and the RF is expressed electromagnetically as ISQ units.
Although Periotest and RFA technologies have shown great promise in dentistry and have helped in adapting and improving implant technologies, they suffer from some drawbacks. The exact correlation of RFA values with bone density or cortical thickness have yet to be clearly established. Periotest technology shows inter-operator and inter-instrument variability. None of these technologies use or provide images of the acceptor site. Most importantly, both technologies allow an assessment of implant stability only after implant insertion or loading, thus limiting post-operative adaptations in the case of improper stability and causing patient discomfort by extended surgery times on implanted bones. They allow a surgeon to check implant integration but do not provide effective and reliable data to predict stability of a planned implant. No such opportunities are available to orthopaedics surgeons.
Implantology professionals use empirical protocols and mean values arising from their own expertise in their practice to design ad hoc implants and implanting surgery protocols. These values fit most surgical situations but do not allow dedicated solutions to out-of-range patients and clinical situations where implants suffer high risk of failure or can cause severe pain, leading to necessary complicated, and most often palliative, surgical interventions. Instead, objective and accurate measurement of predicted implant stability would allow surgeons to make well-informed decisions about implant protocol choices on a case-by-case basis, so that patients could enjoy the benefits of the personalized protocols with higher chances of success.
An Article “Bone density at implant sites and its relationship to assessment of bone quality and treatment outcome” by Bergkvist G, Koh K J, Sahlholm S, Klintström E, Lindh C. in Int J Oral Maxillofac Implants. 2010 March-April; 25(2):321-8 investigates the relationship between bone mineral density (BMD) before implant placement, implant stability measures at implant placement, and marginal bone loss of immediately loaded implants after 1 year in situ. The method uses computed tomographic examination as a preoperative method to assess jawbone density before implant placement. However, after 1 year there were no differences in survival rates or changes in marginal bone density between implants placed in bone tissue of different density. This can be explained in that bone mass or density is not a useful parameter for determining implant stability.
An article (“the JPIS Article”) entitled “A clinical study of alveolar bone quality using the fractal dimension and the implant stability quotient” by Dae-Hyun Lee et al in Journal of Periodontal Implant Science 2010; 40; 19-24—doi: 10.5051/jpis.2010 discusses the evaluation of dental implant stability using fractal analysis to assess bone density. The purpose of this study is to investigate whether the fractal dimension from a panoramic radiograph is related to the primary stability of the implant as represented by RFA. The authors found a linear correlation that was statistically significant between the fractal dimension computed from panoramic X-ray images and ISQ values of RFA. They conclude that the fractal dimension of bone may be a useful method for indicating a general pre-surgical treatment plan. However,
The cited JPIS article is limited to panoramic X-ray images in which the fractal dimension is computed and compared to the Implant Stability Quotient (RFA). The fractal dimension is intended to be a predictor of the sole primary stability. Panoramic X-ray images are however known to be very distorted images and thus not effective for measuring a parameter like the fractal dimension which would be relevant only on images exhibiting scale-invariant spatial properties; a panoramic image cannot have any scale invariant spatial property.